Certain thermoplastic polyurethanes may be processed by blown film techniques, in which a thin tube or "bubble" of polyurethane is most normally extruded upwardly and collapsed by nip rolls at the top of the film tower in which extrusion takes place. Conventional thermoplastic polyurethanes, processed by blown film techniques, are typically highly lubricated to avoid self adhesion (blocking) when the extruded tube is collapsed at the nip rolls, which bring the two thicknesses of plies of the film together. To further reduce the tendency to block, particulate diatomaceous earth is sometimes added to self emboss the blown film surface, thus further reducing contact between the layers. The softer and more elastic the polyurethane, the more surface modification needed. For example, conventional polyurethanes having a Durometer A hardness of 85 and based on 1000 MW polyether or polyester macroglycols require 2 or more parts of wax per 100 parts of polyurethane to avoid blocking.
Certain surface modifying additives are notably undesirable in end use applications requiring subsequent adhesion of the blown film to other materials. An example would be disposable diapers wherein blown polyurethane film is put into strips and adhered to the leg area with a melt adhesive. At high line speeds, excessive surface wax can interfere with adhesion.
U.S. Pat. No. 2,871,218 to Schollenberger illustrates a conventional thermoplastic polyurethane elastomer which can be extruded by blown film techniques. The polyurethanes therein disclosed require a high degree of lubrication, e.g. with 2 to 5 parts of wax per 100 parts of polyurethane, in order to avoid blocking. Linear polyurethane elastomers described in this patent are formed by reacting 1.0 mole of a polyester having molecular weight of 600-1200 with about 0.1 to 2.1 moles of a free glycol containing 4 to 10 carbon atoms and 1.1 to 3.1 moles of a diphenyl diisocyanate, wherein the moles of diisocyanate are equal to the sum of the moles of polyester and free glycol. The polyester is a linear hydroxyl terminated polyester formed by the reaction of a straight chain aliphatic glycol containing some 4 to 10 carbon atoms, such as 1,4-butanediol, with an aliphatic dicarboxylic acid containing from 4 to 10 carbon atoms, such as adipic acid. The free glycol chain extender is also preferably 1,4-butanediol, and more broadly may be an aliphatic-glycol containing from 4 to 10 carbon atoms. While the polyurethane elastomers formed according to this invention have satisfactory tensile strength, high elongation and satisfactory modulus, they do have the tendency to block unless highly lubricated, as already noted.
U.S. Pat. No. 3,001,971 to Scott et al describes another series of thermoplastic polyurethanes which are prepared from a hydroxyl terminated aliphatic polyester, an alkoxydiol and an aromatic diisocyanate in which the --NCO/--OH equivalent ratio is essentially 1:1. Preferred polyesters are as in Schollenberger discussed above, and the preferred alkoxydiol (which serves as chain extender) is 1,4-di(2-hydroxyethyoxy)benzene.
U.S. Pat. No. 3,422,066 to Britain describes a process in which polyurethane fiber or thread having hard and soft segments is prepared by reacting a mixed hydroxyl terminated polyester and a mixture of glycol chain extenders with an excess of organic diisocyanate. The polyester has a molecular weight from 600 to 5000 and is prepared by reacting a mixture of at least 2 saturated aliphatic glycols with a dicarboxylic acid. The chain extender is a mixture of an aromatic glycol, such as 1,4-di(2-hydroxyethoxy)benzene and a straight chain aliphatic glycol.
Also disclosing preparation of a polyurethane elastomer by reaction of a hydroxyl terminated polyester with a chain extender using an excess of organic diisocyanate in U.S. Pat. No. 3,012,992 to Pigott et al. In this case the polyester may be obtained from one dihydric alcohol and one dicarboxylic acid or a mixture thereof. The chain extender or cross linking agent is preferably 1,4-di(2-hydroxyethoxy)benzene. The polyurethane elastomers of this patent are disclosed as being useful in load bearing applications.
The patents cited above are representative of the state of the art on thermoplastic polyurethane elastomers, but this list is by no means exhaustive. It is apparent that physical properties of the products vary widely depending on the specific reactant materials used and the proportions thereof.
Blocking resistance at low lubricant levels, low permanent set after stretching and moderate to high tear propagation resistance are highly desirable properties in any polyurethane elastomer which is to be extruded via blown film technique for elastic tape application. Any two of these three properties can be satisfied by a number of thermoplastic polyurethane compositions. However, it has not been possible to satisfy all three characterists prior to the present invention without resorting to excessive quantities of lubricant, which has the disadvantage noted above with regard to high speed processing.